1. Field of the Invention
The present invention generally relates to vehicle battery temperature control. More specifically, the present invention relates to a vehicle battery temperature control system and method that use liquid phase and vapor phase coolant to maintain desired battery temperature, and add coolant to enhance cooling as necessary.
2. Background Information
A hybrid electric vehicle (HEV) or full electric vehicle relies substantially or completely on battery power for operation. Therefore, it is desirable to maintain the battery cells at an optimal operating temperature. As understood in the art, battery cells are often best suited to operate in a somewhat small optimum temperature range. Consequently, the life or durability of the battery, as well as the performance of the battery, such as the available power provided by the battery, can be adversely affected by temperatures above and below that optimum temperature range. Furthermore, to optimize battery performance, it is essential to maintain temperature uniformity among the cells of a battery, for example, multiple battery cells assembled in a battery module.
Typically, batteries of HEVs or full electric vehicles can be cooled by air, or by a liquid coolant that, for example, has a high water content. However, when cooling fluid is provided by a pumping mechanism, such as a fan or liquid pump, in a single phase (e.g., a liquid phase), temperature gradients will exist along the flow path. That is, because heat is transferred at all points along the flow path, the temperature of the liquid coolant increases from the entrance to the exit of the battery assembly. These gradients can be somewhat reduced by increasing fluid flow rate, which consumes greater energy. Also, shortened parallel flow paths along the cells can be used instead of a serial flow path.
Some systems use the vehicle's air-conditioning (A/C) system refrigerant to cool the battery structure as a primary fluid, or through a secondary fluid which is commonly referred to as a chiller design. Typically, the coldest temperature of the A/C cycle is used to provide the cooling effect. This temperature is generally around 5 degrees Celsius, which is lower than the desired operating temperature range of the battery. Accordingly, a control system is required that, for example, modulates or pulses the flow of cold refrigerant into the battery assembly so as not to overcool the battery. However, this flow modulation can result in the coolant departing from the saturated state, and can create large and detrimental temperature gradients within the battery assembly, thus degrading the battery's life and performance. Furthermore, when the local refrigerant becomes superheated vapor, its temperature will rise due the added heat, thus decreasing the cooling capability. Also, a chiller design will likely have a fluid in the liquid state that will similarly increase in temperature with the added heat.